Vertical and directional gyroscopes, hereinafter referred to as gyros, have been used in aircraft to provide signals to stabilize flight and to provide heading and aircraft attitude information. Typically the two gyros are mounted on a common outer gimbal which pivots on the roll axis of the aircraft. U.S. pat. Nos. 2,941,306 and 3,045,356 illustrate two systems that are illustrative of the state of the art. The Bendix Corporation has developed automatic control and attitude reference systems for aircraft, typical of which is an automatic control system disclosed in U.S. Pat. No. 3,075,729. Of particular interest to this invention is a two-gyro platform manufactured by the Bendix Corporation, Part Number 19005-1B. The outer roll gimbal common to both the vertical and the directional gyro is pivoted by a servomechanism on the roll axis in response to signals from the vertical gyro. This aligns the directional gyro to the vertical gyro gimbals and keeps the directional gyro oriented to the vertical at all times. The outer roll gimbal becomes the third degree of freedom of the vertical gyro.
The roll servo input signal is derived from the syncho transmitter on the roll axis of the outer gimbal common to both gyros. The pitch servo input signal is derived from the synchro transmitter on the pitch axis of the vertical gyro in conjunction with a control transformer on the pitch axis of the directional gyro. These signals are supplied to separate servo motor generators mounted on the gyro platform fixed to the aircraft main frame. These motor generators drive their respective gear trains into a directional gear. The input gear in the roll train to the differential also acts as a direct transmission to the gear that is attached to, and drives, the common roll gimbal. The gear trains are designed so that a pure roll input will produce no relative rotation of the common outer roll gimbal, but when a simultaneous pitch input occurs, the combined inputs will then produce a corresponding rotation of the pitch gimbal to maintain the directional gyro oriented orthogonal to the vertical gyro, i.e., to maintain the third gimbal of the directional gyro normal to the second gimbal of the vertical gyro.
For high performance maneuvers, there is an optional "special functions" operation of this two-gyro four-gimbal attitude sensing system which provides for continuous roll axis information when the aircraft nears vertical pitch. At such an attitude, the roll axis becomes essentially identical with the azimuth axis, so without stabilization, the outer roll gimbal could revolve about the axis of the vertical gyro as a result of the familiar "gimbal lock" phenomenon. As a consequence, there is provided means for sensing when the aircraft pitch nears vertical (within 15.degree.), a special azimuth and roll-pitch resolver signal is fed to the roll servomechanism, causing the outer roll gimbal to maintain its position in space relative to the azimuth of the aircraft immediately preceding the maneuver. This prevents "gimbal lock" and allows for continuous roll information during the maneuver. This is because the common outer roll gimbal is now fixed in space, and not fixed to the airframe, thus permitting true roll indication at any point regardless of the near vertical pitch attitude.
The problem with this sytem for avoiding "gimbal lock" is that geographic heading information is unavailable during a near vertical maneuver. Yet such information is vitally important if the aircraft is to pull out of the maneuver at a desired geographic heading. It would be desirable to modify such a two-gyro four-gimbal system to optionally provide heading information continuously throughout a flight profile whereby the aircraft rotates from a level attitude into a vertical flight attitude, and while in vertical flght, spins about its longitudinal axis, and returns to level flight attitude with normal geographic heading accuracy maintained. More specifically, it would be desirable to provide an electronic signal from a two-gyro four-gimbal attitude system platform that functions to define an aircraft body axis rotation around the aircraft longitudinal axis for onboard cockpit display, or for remote ground based control cockpit display of heading for precise geographic profile control.